Summary

In this study for the Northern Mississippi Valley Loess Hills a procedure was developed for comparing recent water-related soil erosion conditions with the severe conditions described in the nationwide Reconnaissance Erosion Survey of 1934 (RES). The RES led in large part to the establishment of the conservation agencies and programs of today.

Sheet & rill erosion rates per acre and for all land in principal crops in the base year 1930 were estimated using the Universal Soil Loss Equation and then compared with rates estimated for 1982 and 1992 from USDA's National Resources Inventory. Relationships of erosion to livestock and crop production enterprises, soils, rainfall, and farming methods in 1930 were examined for five sample counties.

Results for 1930 for the sample counties were then extrapolated to all 28 counties predominantly in MLRA 105, the Northern Mississippi Valley Loess Hills. The average annual rate of soil loss in 1930 on the land in row crops, small grains and rotation meadow in the region was estimated to have been 14.9 tons per acre per year, plus or minus an allowance for error of 1.0 ton per acre (6.7%). There is a 95-percent level of confidence that the actual rate in 1930 was somewhere in the range 13.9 to 15.9 tons/ac/yr. By 1992 the average rate of soil loss on land in these three crop groups in the region had been reduced to 6.3 tons per acre per year. This represents a 58-percent decrease from the 1930 rate. The allowance for error in this estimate is about 0.3 ton per acre (4.8%). About 80 percent of the soil saved per acre in principal crops between 1930 and 1992 can be attributed to the progress of conservation in the area from 1930 to 1982, and 20 percent from 1982 to 1992.

When multiplied by the acres in principal crops the reduced gross erosion rates per acre from 1930-1992 translate into a reduction of between 42 and 58 percent in the total amount of erosion occurring on the land in row crops, small grains, and meadow. In 1930 between 54 and 64 million tons of soil per year were being displaced from cropland. By 1992 these losses had been reduced to between 27 and 31 million tons per year. About 64 percent of the reduction in total erosion between 1930 and 1992 was achieved by 1982, and 36 percent since 1982. This considers both the reductions in the erosion rate per acre and changes in cropland used.

Background and Objectives

This study analyzes changes in soil erosion conditions between 1930 and 1992 in a fairly large region of the United States, the "Driftless Area of the Northern Mississippi Valley", formally known as Major Land Resource Area No. 105, the Northern Mississippi Valley Loess Hills. The region covers an area of 18,860 square miles (48,847 square kilometers), and includes all or the major part of 28 counties--15 counties in southwestern Wisconsin, six counties in southeastern Minnesota, six counties in northeast Iowa, and a single county (Jo Daviess) in the northwest corner of Illinois (figure 1).

** Missing Image **

For purposes of historical comparison it was desirable that the area be one where cropland agriculture, then and now, used a large part of the landscape. We preferred an area with a relativelyrough topography, where water erosion posed a threat on slopes, as opposed to an area of slight relief. Also, because the analysis attempted to assess the probable effectiveness of Federal conservation programs, the area of study preferably would be one where there had been some early conservation initiatives by public agencies. A logical choice was a major land resource area that included some research and demonstration projects.

In 1933 a new Federal agency, the Soil Erosion Service, selected Coon Creek in Wisconsin as the first watershed within which to demonstrate the values of soil conservation measures. In 1935 this agency became the Soil Conservation Ser-vice , now the Natural Resources Conservation Service (NRCS). The Service began working in the Driftless area in 1933 when it located its first demonstration project at Coon Valley, Wisconsin, a 49,400-acre watershed including parts of La Crosse, Monroe and Vernon Counties.1 The SCS staff worked with local farmers to plan conservation measures for their farmland such as strip cropping, contouring, fencing woodland, and controlling gullies and stream bank erosion. At about the same time the U.S. Department of Agriculture (USDA) had established a number of Conservation Experiment Stations across the country, one of which was located at nearby La Crosse, Wisconsin.

The severe conditions that were documented in the early thirties in the Reconnaissance Erosion Survey (RES) and other field studies of the time led by Hugh Hammond Bennett and others resulted in large part in the soil and water conservation research and project programs in place today.2 Despite the numerous public and private erosion control actions taken since the 1930's, questions continue (as proper) on the efficacy of American conservation programs, some of which have been in place for 60-plus years. Determining how effective these conservation programs have been in reducing soil erosion in a broad region like the Driftless area was a main object of this study.3

General Procedure

The Agricultural Census reporting year 1930 (crop season 1929) was chosen as the base year for the study, as reflecting farming methods generally prevailing during the period 1925-1935, the decade prior to when the Reconnaissance Erosion Survey was conducted in 1934. The study is basically a cross-sectional or 'snapshot' comparison of erosion conditions, agricultural production, and conservation activity between the base year 1930 and the years 1982 and 1992 as years for which the most recent information was available--on erosion from USDA's National Resources Inventories, and on land use and crop production primarily from the relevant Censuses of Agriculture, or from State statistical agencies as needed.

Because the methodologies used in the 1934 Reconnaissance Erosion Survey (RES) and the successive NRI's are not comparable, it was necessary to research in some depth the land use and management practices that led to the serious conditions observed in the RES, using information for the decade 1925-35 from contemporary soil surveys, localized erosion studies, agricultural censuses and other sources. Along with relevant soils and climatic data, these observations were used to 'reconstruct' erosion rates for the base year 1930, using the Universal Soil Loss Equation as developed by Wischmeir and Smith of USDA's Agricultural Research Service in their Agriculture Handbook No. 537, hereafter Handbook 537. 4,5

Another preliminary task was to research the development of agriculture in five sample counties, recognizing that each area has its own unique history. Crop and livestock production data for 1930 and 1992 were then compiled for the sample counties and all 28 counties in MLRA 105, to determine whether the sample was valid and obtain some guidelines on the feasibility of quantifying the various factors involved in the Universal Soil Loss Equation (USLE). Farming systems, tillage practices, harvesting methods, and crop residue handling in the decade 1925-1935, were researched in some detail, to determine proper values specifically for the cover-management and conservation practice factors in the USLE.

Selecting Sample Counties

The five counties sampled were not a random sample in a strict sense, but were an unbiased selection in that they 'happened' to be counties in the region for which soil survey, erosion studies and other reports were available covering the decade 1925-1935, or for the five years on either side of the base year 1930. The sample counties were: Clayton County, Iowa; Houston County, Minnesota; Winona County, Minnesota; Crawford County, Wisconsin; and Vernon County, Wisconsin.

The dates of soil or erosion surveys available for the 28 counties in MLRA 105 are identified in figure 1. The first surveys for the sample counties were generally clustered during the period 1925-1935. In different degrees of detail they described customary farming systems and practices of the time.6

Data on crop and livestock production activity in the five sample counties and for the entire 28-county region were compiled for the base year 1930 and then for 1992 to indicate how well the livestock and crop production economies in the sample counties reflect those of the MLRA 105 region as a whole. The land use and related information for the study drew on three important sources of information centered on the base year 1930: (1) The periodic (5-year) Censuses of Agriculture; (2) annual crop reports compiled by State Agricultural Statisticians and the National Agricultural Statistics Services (NASS); and (3) cropping and/or management practices followed by farmers as observed in the field by soil surveyors.7

Livestock, Land Use and Crop Profiles

Figure 2 shows the relative changes between 1930 and 1992 in the numbers of various livestock. Hogs and beef cattle inventories in the area have increased substantially; other than dairy all other classes show large decreases. By 1992 the number of horses had declined to about 21,000 from the nearly 300 thousand reported on farms in 1930. In 1930 about 85 percent of all farms in the area reported an average of 5 horses or mules. The percentages were similar for the five sample counties and MLRA 105 as a whole. In those years much of the hay and other crops was needed to support the work stock. In the 1992 Census of Agriculture only 12 percent of all farms reported having horses, mules or ponies. 8

** Missing Image **

Though generally on the decline, dairying is still a major livestock industry in many MLRA 105 counties and is also concentrated on fewer farms. Sheep and wool production have declined sharply, as have farm flocks of poultry, but commercial poultry sales appear to have increased. The continued growth of the hog industry provides a ready market for local corn production.

Important changes in land use, crop distributions and other farm indicators are shown in figure 3. The number of farms in MLRA 105 and the sample counties fell by about 50 percent between 1930 and 1992. Average farm size in MLRA 105 in 1930 was 156 acres; in 1992 it was 254 acres. The inflation-adjusted (real) value of farmland for the 28 counties in MLRA 105 has almost doubled, from $375 per acre in 1930 to $720 per acre in 1992. Investments in machinery and equipment per farm in real terms were nearly 5 times as large in 1992 ($51,000) as in 1930 ($11,100). Concerning farm tenancy, in 1992 only 12 percent of the harvested cropland was farmed by tenants who farmed none of their own land, compared with 34 percent in 1930. Crops showing large gains between 1930 and 1992 include alfalfa, corn, soybeans and vegetables. Those losing importance were the small grains and tobacco. The farm economy of MLRA 105, as measured by product sales, remains livestock oriented. In 1992, over 80 percent of gross sales were from livestock or their products, compared with about 50 percent in 1930.

** Missing Image **

To determine how representative the land uses patterns in the five sample counties were of the 28-county region in 1930, a paired statistical t-test was made. Two sets of 20 acreages, in 5 row crops, 3 small grains, 5 rotation meadow options and 7 other 'independent' land uses, like pasture and woodlands were compared, expressing each acreage item as a percentage of all cropland harvested in each county group. It was concluded that land uses in 1930 in the five sample counties were a very good representation of land use throughout the 28-county MLRA 105. The similarity in 1930 of the relative distribution of the main crops in the sample counties and the region is evident in figure 4. 9,10

This test and conclusion are important because the distribution of the various crops, associated tillage practices and methods for handling crop residues across the different counties and soils of the region are major determinants of the cover/management factor 'C' in the Universal Soil Loss Equation (USLE).

A is the computed soil loss per unit area, expressed in the units selected for K and for the period selected for R. In practice, these are usually selected so that A is computed in tons per acre per year, but other units can be selected.

R, the rainfall and runoff factor, is the number of rainfall erosion index units, plus a factor for runoff from snowmelt or applied water where significant.

K, the soil erodibility factor, is the soil loss rate per erosion index unit for a specified soil as measured on a unit plot, which is defined as a 72.6-ft length of uniform 9-percent slope continuously in clean-tilled fallow.

L, the slope-length factor, is the ratio of soil loss from the field slope length to that from a 72.6-ft length under identical conditions.

S, the slope-steepness factor, is the ratio of soil loss from the field slope gradient to that from a 9-percent slope under otherwise identical conditions. In practice L and S can be combined as a single topographic factor LS (not a simple product).

C, the cover and management factor, is the ratio of soil loss from an area with specified cover and management to that from an identical area in tilled continuous fallow.

P, the support practice factor, is the ratio of soil loss with a support practice like contouring, stripcropping, or terracing to that with straight-row farming up and down the slope.

In our erosion analysis it was determined that conservation measures like terracing, contour farming and strip cropping now common in the area were at best developmental in nature and not practiced appreciably enough in the sample counties or elsewhere in the region to be assigned a factor value of less than 1.0 in the Universal Soil Loss Equation, the value for straight-row farming up and down the slope. Values needed for R, K, L and S in the analysis were provided by soil scientists from climatic and soils data, as contained in Handbook 537 and/or Technical Guides prepared for NRCS field offices.

The distributions of crops and/or rotations across major soils used for crops in a given year or period were used to estimate average values for the cover-management factor C. Several kinds of specific information were needed to derive values for the C factor itself:

Crop Rotations: At least 16 different crop rotations, excluding continuous cropping, were mentioned as used or recommended in MLRA 105 in the 1930's, according to soil survey reports and other publications for the period 1925-1935. All were possible candidates for determining approximate values for the USLE cover-management factor C in the base year 1930. Three rotations involved corn rotated with oats or other small grains, seven involved corn rotated with small grains and meadow, and three involved corn rotated only with meadow. Also considered were cropping sequences where the minor row crops of potatoes, tobacco or vegetables were alternated with corn or small grains.

Crop Yields: Yield levels for the base year 1930 were computed on an 'expected' basis, as 10-year averages from 1925-35, using annual data from State statistical offices if available, otherwise data from the Censuses of Agriculture for 1925,1930 and 1935, or in some cases from observations recorded in the field by soil surveyors during the period.

Tillage Methods: Modern conservation tillage technology did not exist in 1930. The moldboard plow was the main primary tillage tool. Three general tillage alternatives were used at the time: (1) Fall moldboard plowing, with secondary tillage in spring, followed by seasonal cultivation as necessary for corn or other row crops; (2) spring moldboard plowing, secondary tillage and/or cultivation; and (3) spring disking.

Crop Residues: Published sources indicated that removing crop residues for roughage or bedding, or by grazing, was a common but not universal practice in 1930. Five residue management situations were accordingly examined: (1) Harvest for grain, residue left and returned to the soil by moldboard plowing or disking; (2) harvest for silage, remaining stubble returned to the soil by moldboard plowing or disking; (3) harvest for grain, stover or straw removed for roughage or bedding; (4) harvest for grain, residue grazed by hogs; and (5) standing crop grazed by hogs.

Cropstages: 1980s data for planting and harvest dates, following Wischmeir and Smith's Handbook 537 for terminology and the dates of selected crop canopy levels, were available at the Midwest National Technical Center (MNTC) of USDA's Natural Resources Conservation Service (NRCS). These data were adjusted as follows to reflect conditions in 1930:

Cropstage F, Rough Fallow Period: MNTC dates were used with no change.

Cropstage 4, Stubble Period: Soil loss ratios for this period reflect the fact that removal of residues for roughage or bedding was a common practice in 1930. For those systems where corn residues were left on the field, soil loss ratios were from Handbook 537 for Cropstage 4L (residues left partially standing, not shredded or spread). This reflected field conditions following husking by hand or harvest with early mechanical pickers.

Cropland and Crop Classifications

These were developed to help match the soils in each county by land use capability class/subclass to cropping sequence groups. A first consideration was that the various crop rotations followed in 1930 were not uniformly distributed over the soil groups. Rotations having relatively high values for C were more likely followed on the better soils. The rotations having lower C values were more likely followed on soils having greater erosion or other hazards and more limitations for production.12

As shown in table 1(col.1), we assumed that in 1925-35 the rotations involving minor row crops and intensive corn production (Crop Groups A and B combined) would have occurred mainly on the soils in land use capability subclasses I, IIe, and IIw. Group C sequences, generally two-crop small-grain/meadow rotations, were assigned to capability subclasses IVe, IIs, IIIs, and IVs. Three-crop corn/small grain/meadow rotations (Group D) were assigned to the capability subclass IIIe lands. The major land capability classes I, II, III and IV are generally usable for cultivated crops, but may have limitations such as erosion hazards (subclass e), excess wetness (subclass w), soil limitations (subclass s), or climatic limitations (subclass c).13

Approximating the cover-management factors C under conditions in 1930 required that the main crops grown and any rotations followed check closely with the available croppable soils as well as with the number of acres of each crop grown in 1930. Crop acres as published in the 1930 Census of Agriculture were used as the statistical controls.

Sample County Results for 1930 and 1992

After the acres in each combination of soils and rotations were determined the USLE factors for K, L, and S were assigned to each soil or soil complex (map unit). Values for K, L, and S depend on characteristics of the soil map units comprising the various land use capability groups. Values for R for each county were taken from Handbook 537.

As supporting conservation practices such as contour farming and terraces were not in general use in 1930, the support practice factor P was assumed to have a constant value of 1.0. Then applying the Universal Soil Loss Equation:

(a) R x K x LS x C x P = average annual soil loss per acre; then

(b) Average annual soil loss per acre x acres = total average annual soil loss for each combination of soils and rotations; and

(c) The sum of soil losses for all the combinations in step (b) = average annual soil loss, in tons per year for the total acres in crops in the county concerned.

Expected average annual USLE soil erosion rates under 1930 conditions were computed as in (a), (b) and (c) above for each designated soil or soil complex (soil map unit), classified as to land use capability in each of the five sample counties, considering further the crop sequences and rotations considered for each map unit. The results of this process were then pooled for the five sample counties (see table 1). Under the distributions of various soils and crops grown in 1930, the average erosion rate on cropland in principal crops ranged from 9.1 tons/ac/yr in Winona County, Minnesota to 22.4 tons/ac/yr in Crawford County, Wisconsin. In each county the erosion rates for 1930 were greatest for the capability subclasses where erosion was the main limitation (IIe, IIIe, and IVe), regardless of whether these areas were used for row crops or small grain rotations with meadow. The estimated mean gross erosion rate across all soils and crops in the five sample counties was 14.9 tons/ac/yr (standard error = 0.5 tons/ac/yr, relative error = 3.5 percent).

Table 1. Soil loss rates in 1930 by crop groups and land use capability subclasses in five sample counties, MLRA 105

Average Crop groups and LCC1

Share of cropland

Clayton County, IA

Houston County, MN

Winona County, MN

Crawford County, WI

Vernon County, WI

Average for five counties

Percent

---- Estimated soil loss rate in 1930, tons/ac/yr ----

Group AB

34.1

8.2

9.4

7.7

8.0

9.9

8.5

Class I

3.3

3.0

3.0

3.6

3.6

3.4

3.3

Sc IIe

24.8

11.0

11.4

8.4

10.5

11.9

10.0

Sc IIw

6.0

6.2

3.4

2.8

4.6

2.9

4.7

Group C

21.4

9.1

21.4

19.1

22.0

16.1

18.5

Sc IVe

19.6

14.8

22.5

22.5

23.1

16.2

20.1

Sc IIs

0.7

0.7

1.0

0.9

1.0

0.8

0.8

SC IIIs

0.4

0.7

1.5

0.9

1.1

0.5

0.9

SC IVs

0.7

2.6

1.6

1.4

2.1

0.8

2.2

Group D

44.5

20.1

15.3

7.8

30.8

19.9

18.2

SC IIIe

44.5

20.1

15.3

7.8

30.8

19.9

18.2

Averages

100.01

16.3

13.9

9.1

22.4

15.9

14.9

Pct.error2

--

8.3%

8.5%

8.4%

7.0%

5.8%

3.5%

Map units3

number

84

74

81

96

102

437

1. Total area for all groups, crops and land use capability classes: 647,300 acres.
2. Standard error of estimate as percent of the estimated mean soil loss rate for the county.
3. Number of different soils or soil complexes designated on modern soil maps for which erosion rates were estimated from the USLE. Where multiple rotations were considered for a given map unit, the mean of their USLE rates was assigned to the unit involved.

Gross rates of soil loss under 1982 and 1992 conditions across all row crops, small grains and rotation meadow for the sample counties were accessed from the data base for the 1992 National Resources Inventory (NRI). The estimated overall rate for the principal crops was 5.5 tons/ac/yr. This was about 63 percent less than the 14.9 tons/ac/yr for 1930.14 The NRI estimates for 1982 and 1992 are based on USLE factor values for 1,945 NRI sample points in the five counties, or for 16.1 percent of the 12,057 sample points for all of the 28 counties predominantly in MLRA 105.15

Gross Erosion in MLRA 105, 1930, 1982 and 1992

Extrapolating to a regional level the cropland erosion rates for 1930 for the five sample counties recognized that any erosion rates computed from the USLE were themselves sample estimates of the gross erosion rates occurring in 1930 across all 28 counties in the region, in that all of MLRA 105 and minor sections in contiguous MLRA's were viewed as the 'population' for which erosion in 1930 was to be estimated from information obtained in the analysis for the sample counties.

As to the broad changes in land use that occurred between 1930 and 1992, the proportions of cropland used for row crops, small grains and rotation meadow in 1930 were nearly the same for the 28 counties in MLRA 105 combined as for the five sample counties (figure 4). In the region row crops went from 31 percent of the area in the main crops in 1930 to 60 percent in 1992. The proportion in small grains went from 41 percent down to 7 percent. The share for meadow, which had become mostly alfalfa by 1992, rose from 28 percent of all land in principal crops in 1930 to 33 percent in 1992.

** Missing Image **

Results of the analysis of gross erosion conditions in 1992 versus 1930 for the Northern Mississippi Valley Loess Hills are given for all 28 counties in the region in table 2. Notes on the derivation and interpretation of the major items are included. Between 1930 and 1992 there was a drop of 58 percent in the erosion rate, from 14.9 tons/ac/yr down to 6.3 tons/ac/yr. The margins of errors in the two rates are respectively 1.0 ton/ac/yr and 0.3 ton/ac/yr (relative errors 6.7 and 4.5 percent). Between 1930 and 1992 the 'average' reduction in gross erosion per acre was 58 percent. Gross erosion had been reduced by about 33 percent between 1930 and 1982. By 1992 the gross erosion occurring in 1982 had been further reduced, by about 27 percent.

Item Explanations:

Item 1. Area estimates from the 1930 and 1992 Censuses of Agriculture. For Census acres, margins of error in constructing the 95-percent confidence interval refer to nonrespondent error for all cropland harvested, a full-count item. Owing to ambiguities in the published relative standard errors for harvested cropland in 1982, the Census Bureau suggested using for 1982 the more accurate relative errors as published for 1992. All error margins in the table refer to the 95-percent confidence interval.

Item 2. Erosion rates derived from the Universal Soil Loss Equation (USLE). For 1930 the rates are developed from factors for rainfall, soil erodibility, field slope and length, cropping patterns, tillage practices, and residue management practices for 437 soils or soil complexes in five sample counties. Error margins for erosion rates per acre in 1930 based on the standard error of this 437-member series of estimated USLE erosion rates. USLE erosion rates for 1982 and 1992, with their margins of error for constructing 95-percent confidence intervals, from USDA's 1992 National Resources Inventory.

Item 3. Average gross erosion per year estimated as the mid-value of the lower and upper limits in item 4. This may not be the same as the simple product of items 1 and 2.

Item 4. The lower limit of the 95-percent confidence interval for gross erosion per year is the product of [crop acres less its margin of error] times [the erosion rate less its margin of error.] The upper limit is the product of [crop acres plus its margin of error] times [the erosion rate plus its margin of error.]

* The single asterisks identify, at a 95-percent confidence level, the minimum percentage reductions in estimated gross erosion between 1930 and 1982, then between 1930 and 1992, and then between 1982 and 1992. They are obtained by subtracting 100 from the upper limit for 1992 taken as a percentage of the lower limit of estimated erosion for 1930, or for 1982 if the comparison is between 1982 and 1992.

** The double asterisks identify, at a 95-percent confidence level, the maximum percentage reduction in estimated gross erosion between 1930 and 1982, then between 1930 and 1992, and then between 1982 and 1992. They are obtained by subtracting 100 from the lower limit for 1992 taken as a percentage of the upper limit of estimated erosion for 1930, or for 1982 if the comparison is between 1982 and 1992.

The gross erosion rates per acre for 1930, 1982 and 1992 have been multiplied by the acreages in principal crops for the entire region to obtain total tons of soil displaced. For 1930 we estimated that between 54 and 64 million tons of soil per year were being displaced from erosion (table 2). By 1992 this had been reduced to between 27 and 31 million tons per year. At a 95-percent level of confidence, it can be stated that reducing the gross erosion rate to 6.3 tons/ac/yr in 1992 translated into a reduction from 1930 to 1992 of between 42 and 58 percent in the amount of gross erosion occurring on the land used for principal crops, for an average or mid-value reduction of 51 percent. Expressing the changes as range estimates allows for errors inherent in the estimates of crop acreages as well as in estimating erosion rates per acre. The mid-value or 'average' displacement was just under 29 million tons in 1992, compared to nearly 59 million tons per year in 1930.

Productivity-Decreasing Erosion

Also examined was the extent to which erosion in the three periods 1930, 1982 and 1992 could be considered to adversely affect long-term and on-site soil productivity. While any erosion is generally undesirable and regarded as 'excessive', excess erosion from a productivity standpoint was evaluated in this study as the amount by which gross erosion rates per acre exceeded allowable tolerances. The 'excess' rate of erosion was defined as the gross rate of displacement less the rate that can occur without an appreciable loss in soil productivity, and without applying substitute nutrients or other soil additives. For the five sample counties this tolerance or T-value varied between 2 and 5 tons/ac/yr, according to particular soils. 17

The overall excess rate in 1930 for the five sample counties (11.9 tons/ac/yr) and its corresponding gross USLE rate per acre, limited to the cropland eroding at rates greater than T (16.7 tons/ac/yr), were extrapolated to the region in calculating total amounts of gross as well as productivity-decreasing erosion.

Summing up the historical erosion analysis, figure 5 relates three measures of aggregate erosion on cropland in 1930, 1982, and 1992: (1) Gross erosion occurring on all cropland: (2) gross erosion occurring on the cropland eroding at rates greater than T; and (3) the amount of excess erosion occurring on the area included in (2). Between 1930 and 1992 all erosion on all cropland fell by 51 percent, or from 59 to about 29 million tons per year. That on the cropland that had been eroding at rates greater than T in 1930 fell by 40 percent. The tons of erosion causing productivity to decline was reduced by 62 percent between 1930 and 1992, or from about 41 down to 16 million tons per year.

Conservation in MLRA 105

As reported above the expected average annual erosion rate per acre of cropland in principal crops in MLRA 105 as computed from the USLE was reduced by an average of 58 percent between 1930 and 1992, while the average annual tons of erosion per year on land in principal crops was reduced by between 42 and 58 percent (table 3). These reductions occurred despite the area in corn or other row crops in 1992 being about 2.3 times what it was in 1930. It appears that Federal, State and private conservation activities in the region have had a definite effect on reducing erosion losses because, other factors considered equal, erosion losses increase with the area devoted to row crop production.

The reductions in erosion were not accomplished by using land resources less intensively, as by leaving land in small grains or permanent hay meadow instead of growing more row crops. They were partly the result of two primary factors: less intensive tillage and a more intensive application of capital to land, represented by the cost of installing recommended on-farm conservation measures and investing in watershed protection and development projects. Also, the much higher crop yields from improved varieties, fertilization, and pest control mean greater quantities of residues. These are increasingly being retained on the surface for erosion control.

Since 1984 conservation tillage has been adopted rapidly in the sample counties and general region. Data from the Conservation Technology Information Center (CTIC) indicate that, as of 1994, no-till farming had been adopted on about 440,000 acres (12 percent) of the land planted to row crops or small grains, compared to none in 1930 and only 3 percent in 1984. In 1994 mulch or ridge tillage was practiced on just over a million acres (26 percent) of the acres in planted crops. Including all variations, some form of conservation tillage was practiced in the region on nearly 40 percent of the area planted to row crops or small grains in 1994. 18

According to the 1992 NRI, stripcropping and/or terraces were in place on 1.3 million acres of cropland, of which 130 thousand acres were terraced. Terracing has increased about 2 percent annually since 1982 and stripcropping at a slightly lower rate.

The Conservation Reserve Program (CRP) is aimed at retiring highly erodible cropland from production through long-term (10-year) contracts with landowners. Contract files indicate that a cumulative total of nearly 726 thousand acres in the region were in the Conservation Reserve in 1994. The enrollments accounted for roughly 85 percent of all cropland not harvested in the region, and represented about 18 percent of the cropland considered highly erodible. 19

The CRP doubtless has been important in protecting previously farmed land from erosion. The vegetative cover of the CRP areas is likely grass or trees, and thus not included in the cropland area for which we calculated per-acre erosion rates. Between 1982 and 1992 there was a net reduction of 507 thousand acres in the area devoted to principal crops for which we estimated erosion rates, but it was not possible to allocate the reductions in total erosion between 1982 and 1992 specifically to the CRP, because the enrolled land was not necessarily used for crops in 1982.

Limitations and Conclusions

The conservation practices initiated since the 1930's enhance many other resources and values such as wildlife, water quality, and Êsthetic and recreational qualities. We did not attempt to quantify these contributions. Nor did we try to determine the relative contributions of Federal or State agencies and individuals in greatly reducing erosion in the region studied, largely because public conservation policies and programs involve cooperation between landowners and public agencies.

The various reasons why farmers may or may not give soil conservation a high priority in their management plans were not investigated here. Nor were the offsite consequences of soil erosion evaluated here, although we can conclude that erosion on and the sediment traceable to farms has been greatly reduced through the actions of farmers and public agencies.20

Farmers of an earlier day in the region were conservation minded. Few attempted to grow corn continuously and steep slopes were generally left in hay or pasture, although pastures were often overgrazed and otherwise poorly managed. The adverse consequences of farming up and down slopes rather than on the contour, and usually removing and sometimes burning crop residues, were not well understood.

Farmers of today are also conservation minded but their situations and tactics differ. The apparent tendency is to plant row crops wherever feasible, but to install the necessary land improvements like terraces, farm slopes on the contour and minimize tillage operations.

Soil erosion has been greatly reduced since 1930 in the Driftless Area of the Northern Mississippi Valley, but the results of our study do not necessarily apply elsewhere. Agriculture is too dynamic and diverse to warrant such generalizations. Nonetheless, this study does offer a clear corrective to the generalizations that imply that soil erosion has remained static or has worsened since the midst of the Great Depression and the dust bowl days of sixty years ago.

This study was an original effort to quantify soil erosion losses 60-plus years ago across a broad region. The numerical results, while reliable, should not be regarded as exact. The results reflect our best judgement as to which source data, assumptions, and analytical methods to apply to the problem. In this sense our findings can be regarded as reasonably accurate representations of farming and erosion conditions in the Driftless Area in 1930 and up to 1992. Further, the continued conversions to no-till farming and other forms of conservation tillage suggest that the expected average annual erosion rate on cropland in this region as of 1995 is measurably less than the 6.3 tons per acre per year we estimated for 1992.

2U.S. National Resource Planning Board, Land Planning Committee, 1936. Soil Erosion: A Critical Problem in American Agriculture. Part V of the Supplementary Report of the Land Planning Committee. See Sec. V, "Erosion Conditions in the United States", p.23ff and Sec. IX, "Erosion Conditions by States".p.55ff. (Washington,D.C.:GPO,1936)

3The early technical studies of soil conservation dealt mostly with representative farm situations on a with- versus a without conservation level, but not tied to physical measures of soil loss. Two conceptual studies for economic analysis are those of Bunce and Heady with Jensen. See: Bunce, Arthur C. 1942. A Method of Estimating the Economic Effects of Planned Conservation on an Individual Farm. U.S. Department of Agriculture Misc. Pub. No. 463. 28pp; also see Heady, Earl O. and Harald R. Jensen. 1951. The Economics of Crop Rotations and Land Use. Research Bul. No. 383. Iowa Agr. Expt. Sta. 459pp.

5Our study was confined to the analysis of water-related (sheet & rill) erosion on cropland. Apart from cost, the reason for focusing on cropland is that the bulk of the erosion discussed in early soil and erosion surveys for the study area was said to occur on cropland. This does not suggest that soil erosion was not a problem on pasture or woodlands. Actually, the overgrazing of woodlands and pastureland led to serious erosion, particularly gully erosion, on these lands as well as cropland.

7Data on farm numbers, crops grown, livestock numbers, county populations, and income sources were mainly from the Censuses of Agriculture and/or Population: Bureau of the Census. 1927. United States Census of Agriculture: 1925. Part I, The Northern States. 1317pp.; Bureau of the Census. 1931. Fifteenth Census of the United States: 1930. Agriculture: Vol. I Farm Acreage and Farm Values by Townships and Minor Civil Divisions, and Vol. II, Part I: Reports by States, with Statistics for Counties and a Summary for the United States; The Northern States. 1385pp.; Bureau of the Census. 1994. Census of Agriculture: 1992. Vol. I Geographic Area Series, State and County Data. Parts 13 (Illinois), 15 (Iowa), 23 (Minnesota), and 49 (Wisconsin). State crop reprts included: Illinois Agricultural Statistics Service. 1951. Illinois Agricultural Statistics, County Estimates for the 20 years ending January 1,1945. Circular 446, 305pp; also Iowa Agricultural Statistics (formerly Iowa Crop and Livestock Reporting Service. 1978 and 1990. Iowa Crops: County Estimates, Historic Series,1950-77, and Iowa Crop County Estimates: Historical Series, 1975-79. Additional photocopies of county-level crop acreages and yields dating from 1930 provided May 1994 by the State Statistician); and Minnesota Agricultural Statistics Service. 1994. Detailed photocopies of crop acreages and yields from 1930-1975 provided May 17,1994 by George Howse, Deputy Statistician.

8Rural counties often arranged for local farmers to assist in county road work, in exchange for waivers of poll or other taxes, in which case the farmer likely kept two extra teams of horses. Source: Mark Bowman and David Gibney. Interview with George A. Pavelis. Elkader, Iowa, 10 February 1995.

9Assuming that each of the 28 counties in MLRA 105 had an equal chance of being included in either the five sampled or the 23 nonsampled counties (having an equal chance of having soil or erosion surveys done between 1925-35), a t-statistic was used to test the null hypothesis that in 1930 there was no relative difference between the land use patterns of the five 'sampled' and the 23 'nonsampled' counties. The calculated t-statistic, for 19 degrees of freedom, was 0.987, compared to a tabular value of 2.093 for the 95-percent level of confidence. In this case the null hypothesis of there being no substantial difference in the two land use patterns is not rejected.

10The later work of Trimble and Lund in Wisconsin applied the USLE to determine by comparative aerial photography the changes between 1934 and 1975 in erosion, as well as the reductions in reservoir and valley sedimentation associated with land use and management practices in 10 sub-basins totaling about 8 thousand acres within the Coon Creek Basin. See: Trimble, Stanley W. and Steven W. Lund. 1982. Soil Conservation and the Reduction of Erosion and Sedimentation in the Coon Creek Basin, Wisconsin. U.S. Geological Survey Professional Paper No. 1234 (Wahington,D.C.:GPO,1982),35pp.

11Definitions for each USLE variable are taken verbatim from Agriculture Handbook No. 537. Op.cit.,Wischmeir and Smith, p.3.

12This may appear contradictory in that, other factors equal, higher C values mean greater erosion, but recall that corn was grown frequently on the better soils. It typically involved moldboard plowing, clean cultivation and residue removal, all of which left the fields vulnerable to both snowmelt and other erosion.

13For a complete description of the land use capability classification system see Klingebiel, A.A. and P.H. Montgomery. 1961. Land Capability Classification. Soil Conservation Service, U.S. Dept. Agr. Handbook No. 210 (Washington,D.C.:GPO,1961),21pp.

14The NRI estimates for 1982 and 1992 are based on observations at about 800 thousand randomly selected sample points located across the United States. For a review of USDA's earlier Inventories and a detailed explanation of the sampling techniques employed in recent Inventories see: Goebel, J.Jeffery. 1992. "Description of the National Resources Inventory". Appendix A in Agricultural Chemical Use and Ground Water Quality: Where Are the Potential Problem Areas? Unnumbered Report, Soil Conservation Service, U.S. Dept. Agr.,with other cooperators. 65pp.; also see Kellogg, Robert L., Gale W. TeSelle and J. Jeffery Goebel. 1994. "Highlights from the 1992 National Resources Inventory". J. Soil and Water Conservation 49(6):521-527.

15Interestingly, the 1930 Census of Agriculture indicates that 647 thousand acres (also 16.4 percent) of the 3.9 million acres of the land in principal crops in the region in 1930, for which USLE erosion rates were reconstructed, were in the five sample counties. The percentage for 1992 was virtually the same--at 16.5 percent. This not only indicates that the five sample counties were and are quite representative of all 28 counties in the region but also that land use shifts since 1930 have affected most of the counties about the same.

16The error margins and relative errors given are for the 95-percent confidence interval. Divide the margins of error by 1.96 to obtain standard errors of estimated mean erosion rates for the two years 1930 and 1992.

17In discussing the present paper at the Symposium on 20th Century Farm Policies, Pierre Crosson of Resources for the Future, Inc. suggested that the T-value concept may not be a reliable basis on which to associate productivity declines with gross erosion rates. Even on relatively deep loessial soils farmers have substituted fertilizers, etc. to compensate for fertility losses in upper soil horizons, and have shifted to reduced tillage to minimize current erosion and help restore previous losses of organic matter. In essence the concept was more relevant to conditions in the area in the 1930s than presently, and also presently if topsoils are shallow. The loess mantle in the Northern Mississippi Valley Loess Hills (MLRA 105), for example, is relatively thin compared to,say,the Iowa and Missouri Deep Loess Hills (MLRA 107).

18Data from the Conservation Technology Information Center obtained and supplied by Carmen Sandretto of the Economic Research Service, USDA.

19Data on the Conservation Reserve Program supplied by Tim Osborn of the Economic Research Service, USDA.